The Structure and Stratigraphy of the Aiguilles Rouges External Massif
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Page 1 The structure and stratigraphy of the Aiguilles Rouges External Massif, its autochthonous Tertiary sediment cover and overlying Morcles nappe in Emosson (Helvetic Realm of the Western Alps, Switzerland) Michael J. Heap1 1University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, U.K. West face of Petit Dent de Morcles Page 2 Abstract The stratigraphy of Emosson is dominated by three main groups of rocks: the metamorphic basement (Aiguilles Rouge massif), its autochthonous Triassic cover and a large recumbent fold called the Morcles nappe. The metamorphic basement displays low-grade subgreenschist metamorphism and is folded on both the km and metre-scale. It also contains many high angle normal faults that have provided palaeostress plunge and plunge directions of 093 64 for σ1, 203 10 for σ2 and 300 27 for σ3. Foliations are folded with the km-scale folding, therefore the foliations pre-date the folding. These foliations could relate to bedding in the sedimentary rocks before they experienced Variscan metamorphism. The metamorphic basement in Emosson is very variable and comprises of nine formations in total, but it mainly consists of a granitoid gneiss called augen gneiss. The top of the metamorphic basement is signalled by an augen gneiss formation containing a red/pink weathered top marking an unconformity. Triassic sediments (the Trias) unconformably overlie this augen gniess and consist of sandstone, mudstone and a carbonate conglomerate called cargneule. The sandstone is a very mature quartz arenite and contains many dinosaur footprints on one of its beds; the presence of a mature, well-rounded arenite with faintly dipping laminations indicates the sandstone was formed in a beach environment. Two types of mudstone are present: a green less oxidised mudstone and a more oxidised purple mudstone. The different colours do not reflect different parent compositions but whether the mud was waterlogged or not during deposition. The mudstones lack bioturbation, contain starved sandstone ripples and were formed on the floodplain of a sandy braided river. The cargneule can be split up into two types: one which contains angular polygonal hollows once occupied by halite and one which contains clasts derived from the surrounding material. The formation of the cargneule occurred in an evapouritic sabkha environment. The top of the cargneule is a decollement surface as the Morcles nappe, a succession of limestones, marls and shales, was thrust over it resulting in the burial of the underlying sediments and basement. This thrusting caused slight metamorphism in the Trias, especially in the cargneule where folding related to nappe emplacement is present. The Morcles nappe contains many large recumbent folds, the hinges lines of these folds were formed in close proximity to a shear plane resulting in the ‘shear out’ of the hinge lines to form sheath folds. Key words: Aiguilles Rouge Massif; Morcles nappe; sheath folds; Triassic sediments; palaeostress. Page 3 Table of contents 1. Introduction………………………………………………………………………5 Location of the mapping area (Emosson)……………………………………5 Alpine history, formation and geology………………………………………7 A brief history of the Alps…………………………………………...7 Western Alp geology and geological setting of Emosson……………8 Aims, scope and outline of the report…………………………………….....13 2. The Variscan metamorphic basement………………………………………….14 Stratigraphy and rock descriptions…………………………………………..14 The small tunnel feldspar gneiss Formation…………………………16 North of the small tunnel layered metasediment Formation…………17 South of the small tunnel mica schist Formation…………………….19 North of the big tunnel garnet gneiss Formation……………...……..21 North of the big tunnel layered metasediment Formation……...……23 Barrage de Barberine augen gneiss Formation…………………...….25 Barrage de Barberine garnet schist Formation…………………...….27 Combe du Col mica schist Formation…………………………….....29 Tete des Ottans augen gneiss Formation………………………….....30 Structural observations within the metamorphic basement……………….....31 Folding……………………………………………………………….31 Faulting………………………………………………………………33 Other observations…………………………………………………..36 Interpretations of the metamorphic basement……………………………….37 Metamorphic grade…………………………………………………..37 Formation of augen gneiss…………………………………………...38 Different formations of augen gneiss………………………………..38 Folding……………………………………………………………….41 Faulting………………………………………………………………44 Implications of garnets in metamorphic formations…………………48 Other interpretations…………………………………………………48 3. The granite……………………………………………………………………….50 The church white granite Formation…………………………………………50 Stratigraphy and rock descriptions…………………………………..50 Interpretations of the granite…………………………………………52 4. The Trias…………………………………………………………………………53 The signpost sandstone Formation…………………………………………..53 Stratigraphy and rock descriptions…………………………………..53 Dinosaur footprints…………………………………………………..58 Col de Barberine interbedded sandstone and mudstone Member……60 Interpretation of the sandstone features………………………………61 The signpost mudstone Formation…………………………………………...63 Stratigraphy and rock descriptions…………………………………...63 Interpretation of the mudstone features………………………………65 The Col du Barberine cargneule Formation………………………………….66 Stratigraphy and rock descriptions…………………………………...66 Page 4 The river valley white structureless Member………………………71 The river mouth quartz limestone Member………………………...72 Interpretation of the cargneule features…………………………….73 Palaeoenvironmental analysis for the Trias………………………………..75 5. The Morcles nappe……………………………………………………………..77 The Combe des Fonds carbonate nappe Formation………………………..77 Stratigraphy and rock descriptions…………………………………77 Structural observations within the Morcles nappe…………………79 Interpretations of the Morcles nappe……………………………….82 6. Superficial deposits…………………………………………………………….86 7. Geological history………………………………………………………………87 Acknowledgements and references………………………………………………91 Appendix…………………………………………………………………………...93 Page 5 1. Introduction Location of the mapping area (Emosson) Emosson is located in the region of Valais, which is located in the southwest of Switzerland (Fig1.2). Emosson was a small village but has been flooded by the consecutive construction of two dams: the Barrage de Barberine in 1925, which was eventually superseded by the Barrage d’Emosson in 1973 (Fig1.1). Position of the now submerged Barrage de Barberine Barrage d’Emosson Lac d’Emosson Figure 1.1 - Photograph of the barrage d'Emosson and Lac d'Emosson (from www.swisscastles.ch) Figure 1.2 - Map of Switzerland with Valais coloured orange Page 6 Figure 1.3 - Map of Valais showing position of Emosson and Lac d’Emosson; the exact mapping area is the east side of Lac d’Emosson (shaded red) The area under study is on the east side of Lac d’Emosson (Fig1.3) from Six Jeur (2062m), located near the Restaurant de Gueulaz, as far as Tour Salliere (3219m) keeping on the west side of Bel Oiseau (2628m), Fontanabran (2702m) and the Pointes d’Aboillon. Page 7 Alpine history, formation and geology A brief history of the Alps Before the Alps were formed the Hercynian (also known as Variscan or Amorican), a vast mountain belt, covered Europe. During the Permian sediments were deposited unconformably on the eroded basement of this orogen, deposition ensued in the Triassic when mostly carbonate marine rocks were deposited. The Adriatic and Eurasian plates collided following the subduction of the Piemonte Ocean (a branch of the Tethys Ocean that opened due to rifting in the Jurassic), which separated the two continents. This resulted in the subduction of the Eurasian plate under the Adriatic plate and the geology of the Alps thus consists of rocks from both continents and oceanic crust. Subduction of this ocean began in the mid-Cretaceous and was directed down towards the south or southeast; the Adriatic plate therefore overrode the Piemonte Ocean and eventually the Eurasian continental margin (Wheeler, 2003). The Pip or Sliver model is an applicable model for the formation of the Western Alps (Fig. 1.4). Figure 1.4 – The Pip or Sliver model (a) Piemonte Ocean subducted and French continental crust is 'pulled' down by more dense mantle (b) crust weakens and the movement sense is reversed forcing the subducted continental crust back to the surface Page 8 Western Alp geology and geological setting of Emosson The main geological units found in Emosson are the Aiguilles Rouge Massif (one of the External Massifs in the Helvetic Realm), its autochthonous Triassic sedimentary cover (mainly carbonates) and a large recumbent fold called the Morcles nappe. More or less allochthonous after the Alpine events, the External Massifs are the relics of a Variscan continental crust that had formed Carboniferous-Permian horsts, within which updomed Variscan nappes are preserved (von Raumer and Schwander, 1984). Overlying the Morcles Nappe is a thin zone of Ultrahelvetic sediments that were thrust over the Morcles’ Triassic to Tertiary sequence during the embryonic stages of the folding responsible for the formation of the Morcles fold. Overlying the Morcles and Ultrahelvetic units are 2-4km of higher Helvetic nappes (Ardon, Diablerets, Mont Gond and Sublage) and more then 5km of now eroded Préalps nappes (Kirschner, Sharp and Masson, 1995). The cross section below shows the positions of the different Helvetic units (Fig 1.5). Préalps nappes Ultrahelvetics nappes Morcles nappe Aiguilles Rouge massif Figure 1.5 - Cross section showing the relative positions of the different Helvetic units (modified from Kirschner